A major effort is underway to develop an understanding of the structure of biologically important macromolecules and their modes of interaction with their environment, with each other and with various drugs. The central tool in this effort has been macromolecular crystallography which has allowed us to visualize antibodies, t-RNAs, drug-DNA complexes, enzyme structures and complete viruses. One of the limiting aspects of many macromolecular structure determinations is the difficulty of obtaining usable X-ray diffraction data due both to the large amounts of data required and to the sensitivity of molecules and crystals to extended X-ray exposure. For the last several years we have been working on developing "CRAD" (a Crystallographic Area Detector). The detector can provide an increase in collection efficiency as great as several orders of magnitude compared to a diffractometer for large molecules. We have collected useful data on a number of biological macromolecules several of which could not have readily been collected by more conventional techniques. The current facility is limited by a number of factors that we plan to correct with the proposed improvements. These include: a low temperature device for stabilizing crystals that are too unstable to be collected at room temperature; the modification of the X-ray tube and crystal mount so that data can be collected to 1.5A; replacement of the current aluminum entrance window with beryllium that will nearby double the efficiency; and a number of solftware improvements that will allow us to take fuller advantage of certain features of the chamber to improve the quality of the data. The various improvements in the chamber will be undertaken in conjunction with substantial data collection in collaboration with a number of laboratories in the Boston area and outside as availability permits. Currently planned projects include high resolution data collection of tRNA and of a number of DNA-drug complexes, a closely related set of plant toxins, several protein-DNA complexes, and several rather large enzymes. The effects of either natural or designed mutants on these structures will be studied. In the case of clinically used antibodies, they will be examined both alone and bound with their antigens.